1. Field of the Invention
The present invention relates to a method of fabricating a Complementary Metal-Oxide Semiconductor (CMOS) Thin Film Transistor (TFT), and more particularly, the present invention relates to a method of fabricating a CMOS TFT using a reduced number of masks.
2. Description of the Related Art
A Complementary Metal-Oxide Semiconductor (CMOS) Thin Film Transistor (TFT) includes both a P-channel Metal-Oxide Semiconductor (PMOS) Thin Film Transistor (TFT) and an N-channel Metal-Oxide Semiconductor (NMOS) Thin Film Transistor (TFT). Therefore, the CMOS TFT is capable of implementing various circuits and systems that are difficult to be implemented using only PMOS TFTs or NMOS TFTs.
Since the CMOS TFT has a PMOS TFT together with an NMOS TFT, it is necessary for the PMOS TFT to have different channel characteristics than the NMOS TFT, i.e., a threshold voltage of the PMOS TFT has to be different from that of the NMOS TFT.
Therefore, in one method of fabricating a CMOS TFT, a predetermined dosage of impurities are implanted into a semiconductor layer of a PMOS TFT or an NMOS TFT to thereby perform channel doping on the PMOS TFT or the NMOS TFT. As a result, a threshold voltage of the PMOS TFT (or the NMOS TFT) is different from that of the NMOS TFT (or the PMOS TFT).
A method of fabricating a CMOS TFT, in which channel doping is performed only on a semiconductor layer of a PMOS TFT is described below with reference to FIGS. 1A to 1G.
Referring to FIG. 1A, in the method of fabricating a CMOS TFT, a buffer layer 10 and a polysilicon layer are formed on the entire surface of a substrate 1, and the polysilicon layer is patterned using a first mask process to thereby form a semiconductor layer 14 of a PMOS TFT and a semiconductor layer 24 of an NMOS TFT. Then, a gate insulating layer 12 is formed on the entire surface of the substrate 1 where the semiconductor layer 14 of the PMOS TFT and the semiconductor layer 24 of the NMOS TFT are formed.
Referring to FIG. 1B, in the method of fabricating a CMOS TFT, a photoresist pattern 50 covering the semiconductor layer 24 of the NMOS TFT is formed on the substrate 1 where the gate insulating layer 12 is formed by a photolithography process employing a second mask process. Then, a small quantity of impurities, such as phosphorus (P), boron (B), etc. are implanted into the semiconductor layer 14 of the PMOS TFT to form a channel-doped semiconductor layer 14b. 
Referring to FIG. 1C, in the method of fabricating a CMOS TFT, a photoresist pattern 60 covering the entire region of the channel-doped semiconductor layer 14b of the PMOS TFT and covering the entire region of the NMOS TFT except for regions where a source region and a drain region are formed is formed on the substrate 1 where the channel-doped semiconductor layer 14b of the PMOS TFT is formed by a photolithography process employing a third mask. Then, N+ type impurities, such as phosphorus (P), arsenic (As), antimony (Sb), bismuth (Bi), etc. are implanted into the regions where the exposed source and drain regions of the NMOS TFT will be formed using the photoresist pattern 60 as a mask to thereby form source and drain regions 24a and 24c of the NMOS TFT. Afterwards, the photoresist pattern 60 is removed by a stripping process.
Referring to FIG. 1D, in the method of fabricating a CMOS TFT, a gate electrode 13 of the PMOS TFT overlapping a region where the channel-doped channel of the PMOS TFT will be formed, and a gate electrode 23 of the NMOS TFT overlapping a region where a channel of the NMOS TFT will be formed are formed on the substrate 1 where the source and drain regions 24a and 24c of the NMOS TFT are formed using a fourth mask process. The gate electrode 23 of the NMOS TFT is formed to have a narrower width than that of the photoresist pattern 60 for forming the source and drain regions 24a and 24c of the NMOS TFT. Impurities are then implanted into the exposed semiconductor layer 14b of the PMOS TFT and the exposed semiconductor layer 24 of the NMOS TFT using the gate electrode 13 of the PMOS TFT and the gate electrode 23 of the NMOS TFT as masks to thereby define the channel-doped channel 14b of the PMOS TFT overlapping the gate electrode 13 of the PMOS TFT and the channel 24b of the NMOS TFT overlapping the gate electrode 23 of the NMOS TFT. Afterwards, Lightly Doped Drain (LDD) regions 24d of the NMOS TFT are formed between the channel 24b of the NMOS TFT and the source and drain regions 24a and 24c of the NMOS TFT.
Referring to FIG. 1E, in the method of fabricating a CMOS TFT, a photoresist pattern 70 covering the entire regions of the semiconductor layers 24a to 24d of the NMOS TFT and covering the entire region of the PMOS TFT except for regions where a source region and a drain region will be formed is formed on the substrate 1 where the LDD regions 24b of the NMOS TFT are formed using a photolithography process employing a fifth mask. Then, P+ type impurities, such as boron (B), aluminum (Al), gallium (Ga), indium (In), etc. are implanted into the regions where the exposed source and drain regions of the PMOS TFT will be formed using the photoresist pattern 70 as a mask to thereby form the source and drain regions 14a and 14c of the PMOS TFT. Afterwards, the photoresist pattern 70 is removed by the stripping process.
Referring to FIG. 1F, an interlayer insulating layer 32 is formed on the entire surface of the substrate 1 where the source and drain regions 14a and 14c of the PMOS TFT are formed. Then, a source contact hole 36 and a drain contact hole 38 exposing the source and drain regions 14a and 14c of the PMOS TFT and the source and drain regions 24a and 24c of the NMOS TFT are formed through the gate insulating layer 12 and the interlayer insulating layer 32 using a sixth mask process.
Next, as illustrated in FIG. 1G, source and drain electrodes 15 and 16 of the PMOS TFT connected to the source and drain regions 14a and 14c of the PMOS TFT through the source and drain contact holes 36 and 38, and source and drain electrodes 25 and 26 of the NMOS TFT connected to the source and drain regions 24a and 24c of the NMOS TFT through the source and drain contact holes 36 and 38 are formed using a seventh mask process.
As described above, the method of fabricating a CMOS TFT, in which a predetermined dosage of impurities are implanted only into the semiconductor layer of the PMOS TFT requires seven mask processes. This is applied to the method of fabricating a CMOS TFT, in which a predetermined dosage of impurities are implanted only into the semiconductor layer of the NMOS TFT as well. The mask process includes a plurality of processes such as a photoresist photo process, exposure and development processes, an etching process, a photoresist stripping process, etc. Therefore, in the method of fabricating a CMOS TFT that requires the seven mask processes, manufacturing processes are complicated, and the CMOS TFT manufacturing costs are increased.